Cost-efficient high energy density redox flow battery

ABSTRACT

Methods and systems are provided for a redox flow battery system. In one example, the redox flow battery is adapted with an additive included in a battery electrolyte and an anion exchange membrane separator dividing positive electrolyte from negative electrolyte. An overall system cost of the battery system may be reduced while a storage capacity, energy density and performance may be increased.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. patent application Ser.No. 16/536,213, entitled “COST-EFFICIENT HIGH ENERGY DENSITY REDOX FLOWBATTERY”, and filed on Aug. 8, 2019, which claims priority to U.S.Provisional Application No. 62/717,625, entitled “COST-EFFICIENT HIGHENERGY DENSITY REDOX FLOW BATTERY”, and filed on Aug. 10, 2018. Theentire contents of the above-listed applications are hereby incorporatedby reference for all purposes.

FIELD

The present description relates generally to methods and systems for aredox flow battery.

BACKGROUND AND SUMMARY

Redox flow batteries are suitable for grid scale storage applicationsdue to their capability for scaling power and capacity independently, aswell as for charging and discharging over thousands of cycles withreduced performance losses in comparison to conventional batterytechnologies. An all-iron hybrid redox flow battery is particularlyattractive due to incorporation of low cost, earth-abundant materials.The iron redox flow battery (IFB) relies on iron, salt, and water forelectrolyte, thus comprising simple, earth abundant, and inexpensivematerials and eliminates incorporation of harsh chemicals therebyallowing the IFB to impose minimal negative impact on the environment.

However, the inventors herein have recognized that further reduction ofoverall system storage costs may be desirable in order to expand aviable commercial application of the IFB. An increased energy storage tounit cost ratio may be achieved by promoting formation of uniform,crack-free plated layers with an increased thickness at a negativeelectrode of the IFB system. An accessibility and performance of the IFBmay thus be improved.

In one example, the issues described above may be addressed by a redoxflow battery system comprising a battery cell with a positiveelectrolyte and a negative electrolyte, the positive electrolyte incontact with a positive electrode and the negative electrolyte incontact with a negative electrode, a plating additive added to thenegative electrolyte, the plating additive interacting with cations ofthe negative electrolyte and forming complexes that plate onto thenegative electrode in self-assembled monolayers.

In this way, an iron redox flow battery (IFB) system may be manufacturedwith a reduced cost of storage. Decreasing the cost of storage mayinclude incorporating a plating additive in the IFB system to enableformation of thick, uniform, and uninterrupted plated layers on thenegative electrode.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an example redox flow battery systemincluding a battery cell with electrodes and a membrane separator.

FIG. 2 shows an example model of a self-assembled monolayer of platedmaterial for a plating electrode.

FIG. 3 shows a graph depicting an example comparison of a maximumplating current density onto a negative electrode versus electrolytetemperature between a conventional redox flow battery and a redox flowbattery configured to plate material as self-assembled monolayers.

FIG. 4 shows an example of a power module of the redox flow battery.

FIG. 5 shows a chart comparing storage costs of a conventional redoxflow battery system with storage coats of the redox flow battery systemadapted with the anion exchange membrane separator and increased platingthickness.

FIG. 6 shows an example of a method for charging and discharging theredox flow battery system.

FIG. 7 shows a schematic diagram of an example of the battery cell ofthe redox flow battery including a negative electrode compartment, apositive electrode compartment, and the membrane separator.

FIG. 8 shows a first schematic diagram of an example of the negativeelectrode compartment without a plating additive where the negativeelectrode is coated with a cracked coating of plated metal.

FIG. 9 shows a second schematic diagram of an example of the negativeelectrode compartment with the plating additive where the negativeelectrode is coated with crack-free coating of plated metal.

FIG. 10 shows a graph comparing a relationship between reactantconcentration and temperature for a first solution including a redoxactive species and supporting salts and for a second solution includingthe redox active species without supporting salts.

FIG. 4 is shown approximately to scale, however, other dimensions may beused as desired.

DETAILED DESCRIPTION

The following description relates to systems and methods formanufacturing a redox flow battery with reduced cost of storage. Theredox flow battery is shown in FIG. 1 with an integrated multi-chambertank having separate positive and negative electrolyte chambers. Theelectrolyte chambers may be coupled to one or more battery cells, eachcell comprising a negative electrode and a positive electrode. Thepositive and negative electrolytes may be separated within each of theone or more battery cells by a membrane separator that selectivelyallows transport of ions across the separator to maintain charge balanceacross the battery cells. Battery performance may be increased byincorporating an additive into a material of the negative, or plating,electrode. The additive may result in a self-imposed monolayerarrangement of plated material, as illustrated in FIG. 2 by an exampleof a model of self-assembled plated monolayers. When adapted with theadditive-promoted plating as self-assembled monolayers, a redox flowbattery may sustain higher plating current density at lower temperaturescompared to a conventional redox flow battery, as depicted in FIG. 3 .Furthermore, an energy density of a battery electrolyte may be enhancedand battery efficiency improved by implementing an anion exchangemembrane separator in the redox flow battery in addition toincorporation of the additive. The plated negative electrode and theanion exchange membrane separator may be installed in a power module ofthe redox flow battery in a configuration shown in FIG. 4 . Overallsystem costs for a conventional redox flow battery are compared withestimated system costs for the redox flow battery of FIG. 4 in aprophetic chart shown in FIG. 5 . An example of a method for operatingthe redox flow battery is provided in FIG. 6 , showing events occurringduring charging and discharging of the battery when equipped with an AEMand the plating additive. A schematic diagram of a battery cell of theIFB, including positive and negative electrode compartments separated bya membrane separator, is illustrated in FIG. 7 to show a formation ofuniform plated layers around a negative electrode of the battery cellresulting from presence of the additive. In the absence of the additivethe coating of metal on to the negative electrode may include cracks, asshown in a first schematic diagram of the negative electrode compartmentin FIG. 8 , which may lead to degradation of the negative electrode. Byincluding the additive in the negative electrode compartment, formationof the crack-free metal coating may be enabled, as shown in FIG. 9 in asecond schematic diagram of the negative electrode compartment. Byincluding the AEM in the IFB, use of costly supporting salts in the IFBelectrolytes may be precluded which may increase a solubility ofreactants (e.g., redox active species) in the electrolytes. A graphdepicting an effect of the AEM, and concomitant absence of supportingsalts, on reactant solubility is shown in FIG. 10 .

FIGS. 4 and 7-9 show example configurations with relative positioning ofthe various components. If shown directly contacting each other, ordirectly coupled, then such elements may be referred to as directlycontacting or directly coupled, respectively, at least in one example.Similarly, elements shown contiguous or adjacent to one another may becontiguous or adjacent to each other, respectively, at least in oneexample. As an example, components laying in face-sharing contact witheach other may be referred to as in face-sharing contact. As anotherexample, elements positioned apart from each other with only a spacethere-between and no other components may be referred to as such, in atleast one example. As yet another example, elements shown above/belowone another, at opposite sides to one another, or to the left/right ofone another may be referred to as such, relative to one another.Further, as shown in the figures, a topmost element or point of elementmay be referred to as a “top” of the component and a bottommost elementor point of the element may be referred to as a “bottom” of thecomponent, in at least one example. As used herein, top/bottom,upper/lower, above/below, may be relative to a vertical axis of thefigures and used to describe positioning of elements of the figuresrelative to one another. As such, elements shown above other elementsare positioned vertically above the other elements, in one example. Asyet another example, shapes of the elements depicted within the figuresmay be referred to as having those shapes (e.g., such as being circular,straight, planar, curved, rounded, chamfered, angled, or the like).Further, elements shown intersecting one another may be referred to asintersecting elements or intersecting one another, in at least oneexample. Further still, an element shown within another element or shownoutside of another element may be referred as such, in one example.

Hybrid redox flow batteries are redox flow batteries that arecharacterized by the deposition of one or more of the electro-activematerials as a solid layer on an electrode. Hybrid redox flow batteriesmay, for instance, include a chemical that plates via an electrochemicalreaction as a solid on a substrate throughout the battery chargeprocess. During battery discharge, the plated species may ionize via anelectrochemical reaction, becoming soluble in the electrolyte. In hybridbattery systems, the charge capacity (e.g., a maximum amount of energystored) of the redox battery may be limited by the amount of metalplated during battery charge and may depend accordingly on theefficiency of the plating system as well as the available volume andsurface area available for plating.

As shown in FIG. 1 , in a redox flow battery system 10, a negativeelectrode 26 may be referred to as a plating electrode and a positiveelectrode 28 may be referred to as a redox electrode. A negativeelectrolyte within a plating side (e.g., a negative electrodecompartment 20) of a battery cell 18 may be referred to as a platingelectrolyte, and a positive electrolyte on a redox side (e.g. a positiveelectrode compartment 22) of the battery cell 18 may be referred to as aredox electrolyte.

Anode refers to the electrode where electro-active material loseselectrons and cathode refers to the electrode where electro-activematerial gains electrons. During battery charge, the positiveelectrolyte gains electrons at the negative electrode 26; therefore thenegative electrode 26 is the cathode of the electrochemical reaction.During discharge, the positive electrolyte loses electrons; thereforethe negative electrode 26 is the anode of the reaction. Alternatively,during discharge, the negative electrolyte and negative electrode may berespectively referred to as an anolyte and anode of the electrochemicalreaction, while the positive electrolyte and the positive electrode maybe respectively referred to as a catholyte and cathode of theelectrochemical reaction. In contrast, during charge, the negativeelectrolyte and negative electrode may be respectively referred to asthe catholyte and cathode of the electrochemical reaction, while thepositive electrolyte and the positive electrode may be respectivelyreferred to as the anolyte and anode of the electrochemical reaction.For simplicity, the terms positive and negative are used herein to referto the electrodes, electrolytes, and electrode compartments in redoxbattery flow systems.

One example of a hybrid redox flow battery is an all iron redox flowbattery (IFB), in which the electrolyte comprises iron ions in the formof iron salts (e.g., FeCl₂, FeCl₃, and the like), wherein the negativeelectrode comprises metal iron. For example, at the negative electrode26, ferrous ion, Fe²⁺, receives two electrons and plates as iron metalon to the negative electrode 26 during battery charge, and iron metal,Fe⁰, loses two electrons and re-dissolves as Fe²⁺ during batterydischarge. At the positive electrode, Fe⁺ loses an electron to formferric ion, Fe³⁺, during charge, and during discharge Fe³⁺ gains anelectron to form Fe²⁺. The electrochemical reaction is summarized inequations (1) and (2), wherein the forward reactions (left to right)indicate electrochemical reactions during battery charge, while thereverse reactions (right to left) indicate electrochemical reactionsduring battery discharge:

Fe²⁺+2e−↔Fe⁰−0.44 V (Negative Electrode)  (1)

Fe²⁺↔2Fe³⁺+2e−+0.77 V (Positive Electrode)  (2)

As discussed above, the negative electrolyte used in the IFB may providea sufficient amount of Fe²⁺ so that, during charge, Fe²⁺ can accept twoelectrons from the negative electrode to form Fe⁰ and plate onto asubstrate. During discharge, the plated Fe⁰ may then lose two electrons,ionizing into Fe²⁺ and may be dissolved back into the electrolyte. Theequilibrium potential of the above reaction is −0.44 V and thus, thisreaction provides a negative terminal for the desired system. On thepositive side of the IFB, the electrolyte may provide Fe²⁺ during chargewhich loses electron and oxidizes to Fe³⁺. During discharge, Fe³⁺provided by the electrolyte becomes Fe²⁺ by absorbing an electronprovided by the electrode. The equilibrium potential of this reaction is+0.77 V, creating a positive terminal for the desired system.

The IFB provides the ability to charge and recharge its electrolytes incontrast to other battery types utilizing non-regenerating electrolytes.Charge is achieved by applying a current across the electrodes viaterminals 40 and 42. The negative electrode 26 may be coupled viaterminal 40 to the negative side of a voltage source so that electronsmay be delivered to the negative electrolyte via the positive electrode(e.g., as Fe²⁺ is oxidized to Fe²⁺ in the positive electrolyte in thepositive electrode compartment 22). The electrons provided to thenegative electrode 26 (e.g., plating electrode) can reduce the Fe²⁺ inthe negative electrolyte to form Fe⁰ at the plating substrate, causingit to plate onto the negative electrode 26.

Discharge can be sustained while Fe⁰ remains available to the negativeelectrolyte for oxidation and while Fe²⁺ remains available in thepositive electrolyte for reduction. As an example, Fe²⁺ availability canbe maintained by increasing the concentration or the volume of thepositive electrolyte to the positive electrode compartment 22 side ofcell 18 to provide additional Fe²⁺ ions via an external source, such asan external positive electrolyte tank 52. More commonly, availability ofFe⁰ during discharge may be an issue in IFB systems, wherein the Fe⁰available for discharge may be proportional to the surface area andvolume of the negative electrode substrate as well as the platingefficiency. Charge capacity may be dependent on the availability of Fe²⁺in the negative electrode compartment 20. As an example, Fe²⁺availability can be maintained by providing additional Fe²⁺ ions via anexternal source, such as an external negative electrolyte chamber 50 toincrease the concentration or the volume of the negative electrolyte tothe negative electrode compartment 20 side of cell 18.

In an IFB, the positive electrolyte comprises ferrous ion, ferric ion,ferric complexes, or any combination thereof, while the negativeelectrolyte comprises ferrous ion or ferrous complexes, depending on thestate of charge of the IFB system. As previously mentioned, utilizationof iron ions in both the negative electrolyte and the positiveelectrolyte allows for utilization of the same electrolytic species onboth sides of the battery cell, which can reduce electrolytecross-contamination and can increase the efficiency of the IFB system,resulting in less electrolyte replacement as compared to other redoxflow battery systems.

Plating quality at the negative electrode 26 may impact a performance ofthe battery system 10. For example, iron may plate from electrolyte ontoa surface of the negative electrode 26 according to electrolyte currentflow. As a result, deposition of iron may be pronounced on features suchas corners, bends or protrusions. Uneven plating may lead to loss ofbattery efficiency and/or capacity by forming gaps along the surface ofthe negative electrode 26 where iron is less likely to plate, an effectthat becomes further exacerbated by continued accumulation of iron metalonto the protruding features or corners or bends. A uniform depositionof iron onto the negative electrode surface throughout the chargingcycle of the battery system 10 may allow for rapid charging anddischarging. Increasing a plating thickness of the iron deposited ontothe negative electrode 26 may enable prolonged energy storage for thedischarge cycle occurring during coupling of the battery system 10 to anelectrically powered external device or system. Furthermore, aconsistent distribution of iron across the electrode surface may provideeven heat distribution, thereby simplifying thermal management of thebattery system 10 and leading to faster charging and discharging of thebattery system 10 at ambient temperature. Methods to promote uniformiron plating via additive-assisted self-assembly of monolayers will bediscussed further below with reference to FIGS. 2, 4-6 .

Efficiency losses in an IFB may result from electrolyte crossoverthrough a separator 24 (e.g., ion-exchange membrane barrier,micro-porous membrane, and the like). For example, ferric ions in thepositive electrolyte may be driven toward the negative electrolyte by aferric ion concentration gradient and an electrophoretic force acrossthe separator. Subsequently, ferric ions penetrating the membranebarrier and crossing over to the negative electrode compartment 20 mayresult in coulombic efficiency losses. Ferric ions crossing over fromthe low pH redox side (e.g., more acidic positive electrode compartment22) to high pH plating side (e.g., less acidic negative electrodecompartment 20) may result in precipitation of Fe(OH)₃. Precipitation ofFe(OH)₃ may degrade the separator 24 and cause permanent batteryperformance and efficiency losses. For example, Fe(OH)₃ precipitate maychemically foul the organic functional group of an ion-exchange membraneor physically clog the small micro-pores of an ion-exchange membrane. Ineither case, due to the Fe(OH)₃ precipitate, membrane ohmic resistancemay rise over time and battery performance may degrade. Precipitate maybe removed by washing the battery with acid, but the constantmaintenance and downtime may be disadvantageous for commercial batteryapplications. Furthermore, washing may be dependent on regularpreparation of electrolyte, contributing to additional processing costsand complexity. Alternatively, adding specific organic acids to thepositive electrolyte and the negative electrolyte in response toelectrolyte pH changes may mitigate precipitate formation during batterycharge and discharge cycling without driving up overall costs.Additionally, implementing a membrane barrier that inhibits ferric ioncross-over may also mitigate fouling.

Additional coulombic efficiency losses may be caused by reduction of H⁺(e.g., protons) and subsequent formation of H₂ (e.g., hydrogen gas), andthe reaction of protons in the negative electrode compartment 20 withelectrons supplied at the plated iron metal electrode to form hydrogengas.

The IFB electrolyte (e.g., FeCl₂, FeCl₃, FeSO₄, Fe₂(SO₄)₃, and the like)is readily available and can be produced at low costs. The IFBelectrolyte offers higher reclamation value because the same electrolytecan be used for the negative electrolyte and the positive electrolyte,consequently reducing cross contamination issues as compared to othersystems. Furthermore, owing to its electron configuration, iron maysolidify into a generally uniform solid structure during plating thereofon the negative electrode substrate. For zinc and other metals commonlyused in hybrid redox batteries, solid dendritic structures may formduring plating. The stable electrode morphology of the IFB system mayincrease the efficiency of the battery in comparison to other redox flowbatteries. Further still, iron redox flow batteries reduce the use oftoxic raw materials and can operate at a relatively neutral pH ascompared to other redox flow battery electrolytes. Accordingly, IFBsystems reduce environmental hazards as compared with all other currentadvanced redox flow battery systems in production.

Continuing with FIG. 1 , a schematic illustration of the redox flowbattery system 10 is shown. The redox flow battery system 10 maycomprise the redox flow battery cell 18 fluidly connected to amulti-chambered electrolyte storage tank 110. The redox flow batterycell 18 may generally include the negative electrode compartment 20,separator 24, and positive electrode compartment 22. The separator 24may comprise an electrically insulating ionic conducting barrier whichprevents bulk mixing of the positive electrolyte and the negativeelectrolyte while allowing conductance of specific ions therethrough.For example, the separator 24 may comprise an ion-exchange membraneand/or a microporous membrane.

In some examples, the separator 24 may be formed from an anion exchangemembrane that conducts a target anion, such as Cl⁻, across the separator24 while inhibiting flow of iron cations (Fe²⁺, Fe³⁺) and iron cationcomplexes, e.g., FeCl₄ ⁻. By configuring the battery system 10 with theanion exchange membrane, an amount of electrolyte solution and number ofchemical species may be reduced. In other words, by adapting theseparator 24 as an anion exchange membrane, the electrolytic speciesinclude active materials such as FeCl₂ and FeCl₃ without additionalsupporting redox inactive electrolytes. In another example, by adaptingthe separator 24 as an anion exchange membrane, the electrolytic speciesmay include only active materials such as FeCl₂ and FeCl₃, which, whendissolved in aqueous solution, provides iron cations that undergo redoxreactions as well as chloride anions for charge balance, withoutadditional supporting redox inactive electrolytes. As a result, a volumeof electrolyte may be reduced while a concentration of active materialsis increased, allowing for a smaller and less expensive storage tank tobe used. Further details of the anion exchange membrane separator willbe provided with reference to FIGS. 3-6 .

The negative electrode compartment 20 may comprise the negativeelectrode 26, and the negative electrolyte may comprise electroactivematerials. The positive electrode compartment 22 may comprise thepositive electrode 28, and the positive electrolyte may compriseelectroactive materials. In some examples, multiple redox flow batterycells 18 may be combined in series or parallel to generate a highervoltage or current in a redox flow battery system. Further illustratedin FIG. 1 are negative and positive electrolyte pumps 30 and 32, bothused to pump electrolyte solution through the flow battery system 10.Electrolytes are stored in one or more tanks external to the cell, andare pumped via negative and positive electrolyte pumps 30 and 32 throughthe negative electrode compartment 20 side and the positive electrodecompartment 22 side of the battery, respectively.

The redox flow battery system 10 may also include a first bipolar plate36 and a second bipolar plate 38, each positioned along a rear-facingside, e.g., opposite of a side facing the separator 24, of the negativeelectrode 26 and the positive electrode 28, respectively. The firstbipolar plate 36 may be in contact with the negative electrode 26 andthe second bipolar plate 38 may be in contact with the positiveelectrode 28. In other examples, however, the bipolar plates may bearranged proximate but spaced away from the electrodes within therespective electrode compartments. The IFB electrolytes may betransported to reaction sites at the negative and positive electrodes 26and 28 by the first and second bipolar plates 36 and 38, resulting fromconductive properties of a material of the bipolar plates 36, 38.Electrolyte flow may also be assisted by the negative and positiveelectrolyte pumps 30 and 32, facilitating forced convection through theredox flow battery cell 18. Reacted electrochemical species may also bedirected away from the reaction sites by the combination of forcedconvection and the presence of the first and second bipolar plates 36and 38.

As illustrated in FIG. 1 , the redox flow battery cell 18 may furtherinclude negative battery terminal 40, and positive battery terminal 42.When a charge current is applied to the battery terminals 40 and 42, thepositive electrolyte is oxidized (lose one or more electrons) at thepositive electrode 28, and the negative electrolyte is reduced (gain oneor more electrons) at the negative electrode 26. During batterydischarge, reverse redox reactions occur on the electrodes. In otherwords, the positive electrolyte is reduced (gain one or more electrons)at the positive electrode 28, and the negative electrolyte is oxidized(lose one or more electrons) at the negative electrode 26. Theelectrical potential difference across the battery is maintained by theelectrochemical redox reactions in the positive electrode compartment 22and the negative electrode compartment 20, and may induce a currentthrough a conductor while the reactions are sustained. The amount ofenergy stored by a redox battery is limited by the amount ofelectro-active material available in electrolytes for discharge,depending on the total volume of electrolytes and the solubility of theelectro-active materials.

The flow battery system 10 may further comprise the integratedmulti-chambered electrolyte storage tank 110. The multi-chamberedstorage tank 110 may be divided by a bulkhead 98. The bulkhead 98 maycreate multiple chambers within the storage tank so that both thepositive and negative electrolyte may be included within a single tank.The negative electrolyte chamber 50 holds negative electrolytecomprising electroactive materials, and the positive electrolyte chamber52 holds positive electrolyte comprising electroactive materials. Thebulkhead 98 may be positioned within the multi-chambered storage tank110 to yield a desired volume ratio between the negative electrolytechamber 50 and the positive electrolyte chamber 52. In one example, thebulkhead 98 may be positioned to set the volume ratio of the negativeand positive electrolyte chambers according to the stoichiometric ratiobetween the negative and positive redox reactions. The figure furtherillustrates the fill height 112 of storage tank 110, which may indicatethe liquid level in each tank compartment. The figure also shows gashead space 90 located above the fill height 112 of negative electrolytechamber 50, and gas head space 92 located above the fill height 112 ofpositive electrolyte chamber 52. The gas head space 92 may be utilizedto store hydrogen gas generated through operation of the redox flowbattery (e.g., due to proton reduction and corrosion side reactions) andconveyed to the multi-chambered storage tank 110 with returningelectrolyte from the redox flow battery cell 18. The hydrogen gas may beseparated spontaneously at the gas-liquid interface (e.g., fill height112) within the multi-chambered storage tank 110, thereby precludinghaving additional gas-liquid separators as part of the redox flowbattery system. Once separated from the electrolyte, the hydrogen gasmay fill the gas head spaces 90 and 92. A such, the stored hydrogen gascan aid in purging other gases from the multi-chamber storage tank 100,thereby acting as an inert gas blanket for reducing oxidation ofelectrolyte species, which can help to reduce redox flow batterycapacity losses. In this way, utilizing the integrated multi-chamberedstorage tank 110 may forego having separate negative and positiveelectrolyte storage tanks, hydrogen storage tanks, and gas-liquidseparators common to conventional redox flow battery systems, therebysimplifying the system design, reducing the physical footprint of thesystem, and reducing system costs.

FIG. 1 also shows the spill over-hole 96, which creates an opening inthe bulkhead 98 between gas head spaces 90 and 92, and provides a meansof equalizing gas pressure between the two chambers. The spill over hole96 may be positioned a threshold height above the fill height 112. Thespill over hole further enables a capability to self-balance theelectrolytes in each of the positive and negative electrolyte chambersin the event of a battery crossover. In the case of an all iron redoxflow battery system, the same electrolyte (Fe²⁺) is used in bothnegative and positive electrode compartments 20 and 22, so spilling overof electrolyte between the negative and positive electrolyte chambers 50and 52 may reduce overall system efficiency, but the overall electrolytecomposition, battery module performance, and battery module capacity aremaintained. Flange fittings may be utilized for all piping connectionsfor inlets and outlets to and from the multi-chambered storage tank 110to maintain a continuously pressurized state without leaks. Themulti-chambered storage tank 110 can include at least one outlet fromeach of the negative and positive electrolyte chambers, and at least oneinlet to each of the negative and positive electrolyte chambers.Furthermore, one or more outlet connections may be provided from the gashead spaces 90 and 92 for directing hydrogen gas to rebalancing reactors80 and 82.

Although not shown in FIG. 1 , integrated multi-chambered electrolytestorage tank 110 may further include one or more heaters thermallycoupled to each of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52. In alternate examples, only one of the negativeand positive electrolyte chambers may include one or more heaters. Inthe case where only the positive electrolyte chamber 52 includes one ormore heaters, the negative electrolyte may be heated by transferringheat generated at the battery cells of the power module to the negativeelectrolyte. In this way, the battery cells of the power module may heatand facilitate temperature regulation of the negative electrolyte. Theone or more heaters may be actuated by the controller 88 to regulate atemperature of the negative electrolyte chamber 50 and the positiveelectrolyte chamber 52 independently or together. For example, inresponse to an electrolyte temperature decreasing below a thresholdtemperature, the controller 88 may increase a power supplied to one ormore heaters so that a heat flux to the electrolyte is increased. Theelectrolyte temperature may be indicated by one or more temperaturesensors mounted at the multi-chambered electrolyte storage tank 110,including sensors 60 and 62. As examples, the one or more heaters mayinclude coil type heaters or other immersion heaters immersed in theelectrolyte fluid, or surface mantle type heaters that transfer heatconductively through the walls of the negative and positive electrolytechambers to heat the fluid therein. Other known types of tank heatersmay be employed without departing from the scope of the presentdisclosure. Furthermore, controller 88 may deactivate one or moreheaters in the negative and positive electrolyte chambers 50, 52 inresponse to a liquid level decreasing below a solids fill thresholdlevel. Said in another way, controller 88 may activate the one or moreheaters in the negative and positive electrolyte chambers 50, 52 only inresponse to a liquid level increasing above the solids fill thresholdlevel. In this way, activating the one or more heaters withoutsufficient liquid in the positive and/or negative electrolyte chamberscan be averted, thereby reducing a risk of overheating or burning outthe heaters.

Further still, one or more inlet connections may be provided to each ofthe negative and positive electrolyte chambers 50, 52 from a fieldhydration system (not shown). In this way, the field hydration systemcan facilitate commissioning of the redox flow battery system, includinginstalling, filling, and hydrating the system, at an end-use location.Furthermore, prior to its commissioning at the end-use location, theredox flow battery system may be dry-assembled at a batterymanufacturing facility different from end-use location without fillingand hydrating the system, before delivering the system to the end-uselocation. In one example, the end-use location may correspond to thelocation where the redox flow battery system 10 is to be installed andutilized for on-site energy storage. Said in another way, it isanticipated that, once installed and hydrated at the end-use location, aposition of the redox flow battery system 10 becomes fixed, and theredox flow battery system 10 is no longer deemed a portable, dry system.Thus, from the perspective of a redox flow battery system end-user, thedry portable redox flow battery system 10 may be delivered on-site,after which the redox flow battery system 10 is installed, hydrated andcommissioned. Prior to hydration the redox flow battery system 10 may bereferred to as a dry, portable system, the redox flow battery system 10being free of or without water and wet electrolyte. Once hydrated, theredox flow battery system 10 may be referred to as a wet non-portablesystem, the redox flow battery system 10 including wet electrolyte.

Further illustrated in FIG. 1 , electrolyte solutions typically storedin the multi-chambered storage tank 110 are pumped via negative andpositive electrolyte pumps 30 and 32 throughout the flow battery system10. Electrolyte stored in negative electrolyte chamber 50 is pumped vianegative electrolyte pump 30 through the negative electrode compartment20 side, and electrolyte stored in positive electrolyte chamber 52 ispumped via positive electrolyte pump 32 through the positive electrodecompartment 22 side of the battery.

Two electrolyte rebalancing reactors 80 and 82, may be connected in-lineor in parallel with the recirculating flow paths of the electrolyte atthe negative and positive sides of the battery cell 18, respectively, inthe redox flow battery system 10. One or more rebalancing reactors maybe connected in-line with the recirculating flow paths of theelectrolyte at the negative and positive sides of the battery, and otherrebalancing reactors may be connected in parallel, for redundancy (e.g.,a rebalancing reactor may be serviced without disrupting battery andrebalancing operations) and for increased rebalancing capacity. In oneexample, the electrolyte rebalancing reactors 80 and 82 may be placed inthe return flow path from the positive and negative electrodecompartments 20 and 22 to the positive and negative electrolyte chambers50 and 52, respectively. Electrolyte rebalancing reactors 80 and 82 mayserve to rebalance electrolyte charge imbalances in the redox flowbattery system occurring due to side reactions, ion crossover, and thelike, as described herein. In one example, electrolyte rebalancingreactors 80 and 82 may include trickle bed reactors, where the hydrogengas and electrolyte are contacted at catalyst surfaces in a packed bedfor carrying out the electrolyte rebalancing reaction. In other examplesthe rebalancing reactors 80 and 82 may include flow-through typereactors that are capable of contacting the hydrogen gas and theelectrolyte liquid and carrying out the rebalancing reactions in theabsence a packed catalyst bed.

During operation of the redox flow battery system 10, sensors and probesmay monitor and control chemical properties of the electrolyte such aselectrolyte pH, concentration, state of charge, and the like. Forexample, as illustrated in FIG. 1 , sensors 62 and 60 maybe bepositioned to monitor positive electrolyte and negative electrolyteconditions at the positive electrolyte chamber 52 and the negativeelectrolyte chamber 50, respectively. In another example, sensors 62 and60 may each include one or more electrolyte level sensors to indicate alevel of electrolyte in the positive electrolyte chamber 52 and thenegative electrolyte chamber 50, respectively. As another example,sensors 72 and 70, also illustrated in FIG. 1 , may monitor positiveelectrolyte and negative electrolyte conditions at the positiveelectrode compartment 22 and the negative electrode compartment 20,respectively. Sensors may be positioned at other locations throughoutthe redox flow battery system 10 to monitor electrolyte chemicalproperties and other properties. For example a sensor may be positionedin an external acid tank (not shown) to monitor acid volume or pH of theexternal acid tank, wherein acid from the external acid tank is suppliedvia an external pump (not shown) to the redox flow battery system 10 inorder to reduce precipitate formation in the electrolytes. Additionalexternal tanks and sensors may be installed for supplying otheradditives to the redox flow battery system 10. For example, varioussensors including, temperature, conductivity, and level sensors of afield hydration system may transmit signals to the controller 88.Furthermore, controller 88 may send signals to actuators such as valvesand pumps of the field hydration system during hydration of the redoxflow battery system 10. Sensor information may be transmitted to acontroller 88 which may in turn actuate pumps 30 and 32 to controlelectrolyte flow through the cell 18, or to perform other controlfunctions, as an example. In this manner, the controller 88 may beresponsive to, one or a combination of sensors and probes.

Redox flow battery system 10 may further comprise a source of hydrogengas. In one example the source of hydrogen gas may comprise a separatededicated hydrogen gas storage tank. In the example of FIG. 1 , hydrogengas may be stored in and supplied from the integrated multi-chamberedelectrolyte storage tank 110. Integrated multi-chambered electrolytestorage tank 110 may supply additional hydrogen gas to the positiveelectrolyte chamber 52 and the negative electrolyte chamber 50.Integrated multi-chambered electrolyte storage tank 110 may alternatelysupply additional hydrogen gas to the inlet of electrolyte rebalancingreactors 80 and 82. As an example, a mass flow meter or other flowcontrolling device (which may be controlled by controller 88) mayregulate the flow of the hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110. The integrated multi-chambered electrolytestorage tank 110 may supplement the hydrogen gas generated in redox flowbattery system 10. For example, when gas leaks are detected in redoxflow battery system 10 or when the reduction reaction rate is too low atlow hydrogen partial pressure, hydrogen gas may be supplied from theintegrated multi-chambered electrolyte storage tank 110 in order torebalance the state of charge of the electro-active species in thepositive electrolyte and negative electrolyte. As an example, controller88 may supply hydrogen gas from integrated multi-chambered electrolytestorage tank 110 in response to a measured change in pH or in responseto a measured change in state of charge of an electrolyte or anelectro-active species. For example an increase in pH of the negativeelectrolyte chamber 50, or the negative electrode compartment 20, mayindicate that hydrogen is leaking from the redox flow battery system 10and/or that the reaction rate is too slow with the available hydrogenpartial pressure, and controller 88, in response to the pH increase, mayincrease a supply of hydrogen gas from integrated multi-chamberedelectrolyte storage tank 110 to the redox flow battery system 10. As afurther example, controller 88 may supply hydrogen gas from integratedmulti-chambered electrolyte storage tank 110 in response to a pH change,wherein the pH increases beyond a first threshold pH or decreases beyondsecond threshold pH. In the case of an IFB, controller 88 may supplyadditional hydrogen to increase the rate of reduction of ferric ions andthe rate of production of protons, thereby reducing the pH of thepositive electrolyte. Furthermore, the negative electrolyte pH may belowered by hydrogen reduction of ferric ions crossing over from thepositive electrolyte to the negative electrolyte or by protons,generated at the positive side, crossing over to the negativeelectrolyte due to a proton concentration gradient and electrophoreticforces. In this manner, the pH of the negative electrolyte may bemaintained within a stable region, while reducing the risk ofprecipitation of ferric ions (crossing over from the positive electrodecompartment) as Fe(OH)₃. Other control schemes for controlling thesupply rate of hydrogen gas from integrated multi-chambered electrolytestorage tank 110 responsive to a change in an electrolyte pH or to achange in an electrolyte state of charge, detected by other sensors suchas an oxygen-reduction potential (ORP) meter or an optical sensor, maybe implemented. Further still, the change in pH or state of chargetriggering the action of controller 88 may be based on a rate of changeor a change measured over a time period. The time period for the rate ofchange may be predetermined or adjusted based on the time constants forthe redox flow battery system 10. For example the time period may bereduced if the recirculation rate is high, and local changes inconcentration (e.g., due to side reactions or gas leaks) may quickly bemeasured since the time constants may be small.

As described above, an overall performance of a redox flow battery maybe improved by a combination of uniform, thick, reversible plating ofmetal onto a negative (plating) electrode and presence of an anionexchange membrane separator controlling flow of ions between a positiveelectrode compartment and a negative electrode compartment of a batterycell. In an iron redox flow battery (IFB) system, ferrous iron, Fe²⁺ isreduced to iron metal, Fe⁰, at the negative electrode, resulting indeposition of the metal during the IFB charging cycle. Formation ofcontinuous, uniform, and crack-free plated layers that coat a surface ofthe negative electrode may reduce a likelihood of uneven current densityacross a surface of the negative electrode which may lead to developmentof localized heating that degrades the electrode and diminishes theelectrode lifetime. In addition, as the IFB may undergo tens ofthousands of charge/discharge cycles, maintaining uniform plating overthe cycles is highly desirable.

Furthermore, in order to achieve 100 hours of energy storage in the IFBat a target power level, a desirable plating thickness may be greaterthan 1 cm. The increased plating thickness, as compared to plating inconventional redox flow battery systems, may provide a sufficient amountof plated iron on the negative electrode to continuously oxidizeelemental iron to ferrous ion over a period of 100 hours during batterydischarge. To obtain consistently uniform and reversible plating, anadditive may be used to induce deposition of plated iron inself-assembled monolayers, as shown in FIGS. 2 and 7 .

A small quantity of an additive may be added to a negative electrolytesolution, e.g., the negative electrolyte stored in the negativeelectrode compartment 20 of FIG. 1 , at millimolar (mM) concentrationsso that the presence of the additive does not affect system costs oroverall energy and power density. A model 200 of a self-assembly ofmonolayers with an additive 202 is shown in FIG. 2 . The additive 202may be a fatty acid, such as stearic acid 202, which may interact withan iron center 204 at an electron-rich carboxylate functional end group206 of the stearic acid molecule. For the redox flow battery system 10of FIG. 1 , in the case of an IFB system, the negative (plating)electrode 26 may include an iron substrate (including one or more ironcenter 204). The functional end group 206 may form a chemical bond withthe iron center 204. A long-chain carbon tail 208 of stearic acid 202may trail from the adsorbed functional end group 206, away from the ironcenter 204. The trailing carbon tail 208 may be much longer in lengththan the functional end group 206 of stearic acid 202. For example, thecarbon tail 208 may be a first length 203, such as 2.15 nm, while thefunctional end group 206 may be a second, shorter length 205, such as0.15 nm, as shown in FIG. 2 . Van der Waals interactions between eachstearic acid carbon tail 208 and adjacent carbon tails may result in atight packing arrangement of each layer of plated iron to reduce a freeenergy of each layer. Each consecutive plated layer may be similarlypacked as a new monolayer adjacent to a previously plated monolayer,forming a stack 210 of evenly spaced apart monolayers of iron that areseparated by layers of stearic acid carbon tails surrounding each ironcenter 204. A stacking of monolayers of iron are depicted in a schematicdiagram 700 in FIG. 7 .

The schematic diagram 700 shows a battery cell 702, which may be anon-limiting example of the battery cell 18 of FIG. 1 . The battery cell702 includes a negative electrode compartment 704 with a negativeelectrode 706 submerged in a negative electrolyte 708. The negativeelectrode compartment 704 may be separated from a positive electrodecompartment 710, containing a positive electrode 712 submerged in apositive electrolyte 714, by a membrane separator 716. In the exampleshown in FIG. 7 , the membrane separator 716 is an anion exchangemembrane (AEM) separator 716, allowing only anions 720 to passtherethrough. In other examples, however, the membrane separator 716 maybe configured to be a cation exchange membrane separator or amicroporous substrate.

Both the negative electrolyte 708 and the positive electrolyte 714comprise cations 718 (e.g., Fe²⁺, Fe³⁺), which may each be the ironcenter 204 of FIG. 2 , and anions 720 (e.g., Cl⁻). The negativeelectrolyte 708 may additionally include additive molecules 722, such asthe stearic acid 202 of FIG. 2 , which bind with the cations 718 at oneend to form complexes 724. A number of additive molecules 722 bound toeach of the cations 718 may vary. For example, complexes 724 with one,two, or four bound additive molecules 722 are shown in FIG. 7 butcomplexes with other quantities of bound additive molecules 722, such asthree, five, or six, have been contemplated.

The complexes 724 may plate onto the negative electrode 706 and formconsecutive self-assembled monolayers 726, similar to the stack 210 ofiron monolayers shown in FIG. 2 . Each layer of the monolayers 726 ofiron may have a uniform thickness, defined in a direction perpendicularfrom a surface 728 of the negative electrode 706, along a portion of thesurface 728 of the negative electrode 706 that is in contact with thenegative electrolyte 708, e.g., submerged in the negative electrolyte708. A stacking of uniform monolayers 726 of iron onto the negativeelectrode 706 may allow for a plating thickness 730, also defined in adirection perpendicular to the surface 728 of the negative electrode706, of greater than 1 cm while maintaining a consistent thickness ofthe plating thickness 730 across the surface 728 of the negativeelectrode. The plating thickness 730 may represent a sum of thethicknesses of each layer of the stacked monolayer 726 of iron.

It will be appreciated that while use of stearic acid in the IFB isdescribed, with respect to FIG. 2 , numerous other types of fatty acidsor surfactants may be used to promote self-assembly of the plated ironinto monolayers without departing from the scope of the presentdisclosure. For example, various other additives which include afunctional end group interacting and binding to iron cations, as well asa chemically inert tail formed from a hydrocarbon chain, may be used inplace of stearic acid.

The additive, which may be stearic acid in one example, may promoteself-assembled monolayers that form an overall crack-free coating ofiron on the negative electrode. For example, a first schematic diagram800 of a negative electrode compartment 802, similar to the negativeelectrode compartment 20 of FIG. 1 and 704 of FIG. 7 , is shown in FIG.8 . A portion of a negative electrode 804 is submerged in a firstnegative electrolyte 806 which may contain a mixture of metal cationsand anions in aqueous solution. Upon reduction, the metal may plate ontothe negative electrode 804 forming a coating 808. The coating 808includes cracks 810, which may be gaps in the coating 808 forming as aresult of topographic features or an uneven texture of a surface of thenegative electrode 804.

A presence of cracks 810 in the coating 808 disrupts electricalconductivity through the coating 808. Uneven distribution of current mayresult, causing uneven heating and formation of hotspots along thesurface of the negative electrode 804. Degradation of the negativeelectrode 804 may occur due to the uneven heating.

In contrast, a second schematic diagram 900 shown in FIG. 9 of thenegative electrode compartment 802 may store a second negativeelectrolyte 902. The second negative electrolyte 902 may be a mixture ofmetal cations, anions, and an additive, such as stearic acid. Uponreduction of ferrous iron at the negative electrode 804, the metal mayplate onto the negative electrode 804 in self-assembled monolayers, asdescribed in FIGS. 2 and 7 , resulting in deposition of a smooth,continuous, and crack-free coating 904 of metal. A continuity of currentflow through the coating 904 is maintained, allowing for even heating ofthe negative electrode 804 and preserving and prolonging an integrity ofthe negative electrode 804.

The additive may, in addition to promoting formation of organized andcrack-free layers of plated iron, inhibit corrosion of the iron-platednegative electrode. For example, as shown in FIG. 7 , each iron centermay adsorb more than one stearic acid functional end group and besurrounded by more than one attached stearic acid tail, the plurality ofstearic acid tails around each iron center providing a coating aroundthe iron centers that reduces oxidation of iron during storage or whenthe IFB is in an idle state.

Compared to IFB systems operating without plating additives, theself-assembled monolayer plating of iron, as described above, may allowplating and deplating (e.g., electron exchange between Fe⁰ to Fe²⁺) tooccur at lower temperature, such as ambient temperature, in the IFB whenimplemented with suitable system control logics. Conventional iron flowbatteries may operate at elevated temperatures of 50-65° C., forexample, to achieve high charging efficiency, adding complexity andconsuming energy in order to heat the IFB to provide a desirable poweroutput. Operation of the IFB at ambient temperature rather than highertemperatures may simplify the system and reduce overall costs.

A comparison of plating current density between a conventional IFB andthe IFB configured to plate iron in self-assembled monolayers is shownin a graph 300 in FIG. 3 . Graph 300 depicts a maximum plating currentdensity in mA/cm² increasing upwards along the y-axis and electrolytetemperature increasing to the right along the x-axis. A first plot 302of an IFB with a conventional plating system, e.g., no additive, isgiven in graph 300 along with a second plot 304 of an IFB with anadditive, such as stearic acid, to promote self-assembly of monolayersduring iron plating.

At ambient temperature (T_(amb)), a difference in maximum platingcurrent density between the first plot 302 and the second plot 304 isgreatest. The second plot 304 indicates that the additive-equipped IFBmay support a maximum plating current density that is higher than amaximum plating current density shown by the first plot 302 when equalreversibility and plating quality is demonstrated by both systems. Thedifference in plating current density between the first plot 302 andsecond plot 304 decreases as temperature increases, with the second plot304 consistently higher than the first plot 302 until the plots merge ata terminal high end temperature. The results shown in graph 300 indicatethat uniform, crack-free plating of iron together with pulse chargingallows significantly thicker plating of iron onto a negative electrodeof the IFB without adversely affecting battery performance at relativelylow operating temperatures. A deviation between plating current densityof the second plot 304 versus the first plot 302 is most pronounced atambient temperature. Thus, the system represented in plot 304 maysupport greater plating thickness on the negative electrode withoutcausing significant electrolyte volume change, drop in pressure, or cellclogging over 100 hours of storage.

Furthermore, high efficiency charging of conventional redox flowbatteries may be associated with high plating overpotentials which maylead to hydrogen evolution at the negative electrode. In the IFB systemwith self-assembled monolayers of plated iron, pulse charging may reduceplating overpotential and further assist in uniform iron plating bydecreasing iron grain size. In some example, pulse charging, incombination with the additive, may assist in reducing electrodeoverpotential, thus reducing a likelihood of hydrogen generation at thenegative electrode.

Energy density of the IFB may also be enhanced by implementing an anionexchange membrane (AEM) as a separator between a negative electrodecompartment and a positive electrode compartment. Incorporation of theAEM, in addition to an additive to control iron plating, may alsodecrease storage costs to, for example, $10 per kilowatt hour byeliminating supporting electrolytes, such as KCl, and demonstrategreater than 60% round trip efficiency of the IFB. The AEM may interactwith anions, such as Cl⁻, and not cations, such as Fe²⁺ and Fe³⁺,allowing anion transport across electrode compartments via an anionexchange mechanism. In addition, with application of AEM and eliminationof supporting electrolytes, reactants concentration in water can besignificantly increased, i.e. the overall system energy efficiency canbe increased.

For example, as shown in FIG. 7 , anions 720 may flow across the AEMseparator 716 between the negative electrode compartment 704 andpositive electrode compartment 710 but not the metal cations 718, theadditive molecules 722, or the complexes 724. In one example, the anions720 may be Cl⁻ in an IFB system. As described above, the anions 720 maybe transported across the AEM separator 716 in response to the chargeimbalance in the battery cell 702. For example, generation of H₂ duringIFB operation, as described above with reference to FIG. 1 , may resultin uneven distribution of charges between the positive electrodecompartment 710 and the negative electrode compartment 704. The chargeimbalance may be alleviated by enabling the anions 720 to flow acrossthe AEM separator 716, either in a first direction from the negativeelectrode compartment 704 to the positive electrode compartment 710 asindicated by arrow 703 or in a second direction from the positiveelectrode compartment 710 to the negative electrode compartment 704 asindicated by arrow 705. Anion transport may occur during regular chargeand discharge reaction of the IFB, allowing, for example, only or mostlyCl⁻ movement across the AEM separator 716.

For example, when an overall charge balance of the positive electrodecompartment 710 is biased positive while an overall charge balance ofthe negative electrode compartment 710 is neutral, the anions 720 mayflow along the first direction as indicated by arrow 703 to neutralizethe overall positive charge. Similarly, when the overall charge balanceof the negative electrode compartment 710 is biased positive while theoverall charge balance of the positive electrode compartment 703 isneutral, the anions 720 may flow along the second direction, asindicated by arrow 705. As such, charge balance in the battery cell 702may be restored and stable operation of the IFB is maintained.

Furthermore, implementation of the AEM separator 716 may hindercross-over of iron cations between the positive electrode compartment710 and the negative electrode compartment 704. By inhibiting migrationof iron cations across the AEM separator 716, precipitation of Fe(OH)₃is circumvented and a likelihood of membrane fouling at the AEMseparator 716 is reduced. As well, loss of iron due to Fe(OH)₃ formationmay be mitigated.

Equipping the IFB with the AEM separator may also decrease an overallcost of the IFB. By inhibiting cation flow, use of supportingelectrolyte to increase a conductivity of the solution may beeliminated. In conventional redox flow battery systems, supportingelectrolyte, comprising electrically conductive species in solution thatdo not participate in the redox reactions of the redox flow battery, mayimpose significant additional costs. However, the IFB may be configuredto instead rely on electrolyte containing exclusively electroactivespecies involved in iron redox chemistry when the AEM separator isinstalled, thereby increasing energy density and decreasing systemcosts. A lesser volume of electrolyte may be used with up to a twofoldincrease in concentration of redox active iron species providing asuitable amount of Fe²⁺ to enable an iron plating of greater than 1 cmthickness.

Various types of AEMs may be considered to form the separator. Forexample, a polymer network of the separator membrane may includeheteroaromatic compounds, aniline, olefins, or sulfones as buildingblocks. Alternatively, the membrane may be formed from a covalentorganic framework or include pH resistant functional groups. The AEM maybe fabricated by numerous methods including grafting, surface coating,solvent casting, conformal coating or, as another example, may becommercially available.

Eliminating supporting salts from electrolyte of an IFB system mayresult in increased energy density of the IFB. As described above,implementation of an AEM allows exclusive dissolution of redox activespecies in the electrolyte thereby enhancing a solubility of the redoxactive species in a given volume of electrolyte. As an example,solubility of the redox active species without presence of supportingsalts is compared with solubility of the redox active species withsupporting salts included in an electrolyte in graph 1000, as depictedin FIG. 10 .

Graph 1000 shows a temperature of the electrolyte increasing to theright along the x-axis and a concentration of a reactant or redox activespecies in solution, e.g., an amount of the redox active speciesdissolved in water, increasing upwards along the y-axis. The redoxactive species may be FeCl₂ and/or FeCl₃. A first plot 1002 represents aconcentration of the reactant with supporting salts also dissolved inthe solution and a second plot 1004 represents a concentration of thereactant without presence of the supporting salts. When the temperatureis low, such as at −10° C., a solubility of the reactant may be similarwith or without the presence of the supporting salts.

As the temperature increases, the first plot 1002 and the second plot1004 diverge, with the concentration of the reactant rising more rapidlyin the second plot 1004 than the first plot 1002. Above 0° C.,solubility of the reactant increases in both plots but reactantsolubility is consistently higher in the second plot. For example, in anoperating temperature range of an IFB of between 50-60° C., as indicatedby shaded area 1006, the concentration of the reactant is at least threetimes higher than the reactant concentration when the supporting saltsare present. Thus, the presence of supporting salts may suppressreactant solubility, decreasing an amount of redox active species ableto engage in charge/discharge cycles of the IFB.

The solubility of the redox active species may be directly correlated toan energy density of the IFB. Increasing the solubility of the redoxactive species may result in higher energy density while reducing thesolubility of the redox active species may decrease the IFB energydensity. With respect to graph 1000, the greater solubility of thereactant at temperatures between 50-60° C., as indicated by shaded area1006, may result in a greater than twofold increase in energy density ofthe IFB. Elimination of the supporting salts may thereby enhance anefficiency of the IFB system.

An IFB system may include a power module adapted with both an additiveto encourage uniform plating and an AEM as a separator between anegative electrode and a positive electrode, the AEM having inherent ionselectivity. An example of a power module 400 that may be used in aredox flow battery system, such as the redox flow battery system of FIG.1 , is shown in FIG. 4 . A set of reference axis 401 is provided,indicating a y-axis, an x-axis, and a z-axis. The power module 400comprises a series of components arranged as layers within the powermodule 400. The layers may be positioned co-planar with a y-x plane andstacked along the z-axis.

Pressure plates 402 may be arranged at a first end 403 and a second end405 of the power module 400 that provide rigid end walls that defineboundaries of the power module 400. The pressure plates 402 allow layersof the power module 400 to be pressed together between the pressureplates 402 to seal components of the power module within an interior 407of the power module 400. Picture frames 404 may be arranged inside ofthe pressure plates, e.g., against sides of the pressure plates facinginwards along the z-axis, towards the interior 407 of the power module400, the picture frames 404 adapted to interface with one another toseal fluids within the interior 407 of the power module 400.

Elements of the power module 400 are now described along a directionfrom the first end 403 towards the second end 405. A negative spacer 406is arranged adjacent to one of the picture frames 404 positioned at thefirst end 403, the negative spacer 406 defining flow channels along asurface of a negative electrode. A bipolar plate 408, which may have anintegrated negative electrode along a surface of the bipolar plate 408in face-sharing contact with the negative spacer 406, is positionedbetween the negative spacer 406 and surrounded by a bipolar plate frameplate 410 that provides structural support.

A positive electrode 412, which may be a sheet of graphite felt, isarranged along a face of the bipolar plate 408 opposite of the negativespacer 406. A membrane 414, surrounded by a membrane frame plate 416 forstructural support, may be positioned adjacent to the positive electrode412, on a side of the positive electrode facing the second end 405 ofthe power module 400. The membrane 414 may be adapted as an anionexchange membrane, transporting anions across the membrane 414 but notcations or complexes. The components described above, e.g., the negativespacer 406, the bipolar plate 408, the positive electrode 412, and themembrane 414 may repeat within the power module, from the first end 403to the second end 405, a number of times, forming a battery stack.Negative electrolyte, including an additive such as stearic acid, may becontained between another membrane, arranged on a side of the bipolarplate 408 towards the first end 403 of the power module 400, and thebipolar plate 408, the negative electrolyte in contact with both thenegative spacer 406 and integrated negative electrode (e.g., integratedinto the surface of the bipolar plate 408). Positive electrolyte may becontained between the bipolar plate 408 and the membrane 414, in contactwith the positive electrode 412.

Estimated reductions in system costs resulting from adapting an IFB withan AEM and an additive for uniform iron plating is compared to costs ofa conventional IFB in a prophetic chart 500 illustrated in FIG. 5 .System costs in dollars per kilowatt hour is shown increasing upwardsalong a y-axis of chart 500 over a 100 hour storage period at a ratedpower. A first column 502 shows a system cost breakdown for aconventional IFB. A majority of system costs may be attributed toelectrolyte, represented by a first hatched area 504 in the first column502. In a second column 506, a majority portion of a system cost of anAEM and additive-adapted IFB is similarly attributable to electrolyteand represented by a second hatched area 508. The electrolyte mayinclude both redox active species and supporting salts. However, theamount due to electrolyte in the second column 506 is reduced comparedto the first column 502, indicating a cost savings of, for example, 30%.The lower electrolyte costs for the second column 506 may results from adecreased volume of electrolyte in the IFB when the AEM is implemented,as described above. The decreased volume of electrolyte may increase anenergy density of the electrolyte by increasing a concentration of theredox active species such as FeCl₂ and FeCl₃. In addition, use ofexpensive supporting salts, e.g., electrically conductive species thatdo not participate in redox reactions of the IFB, may be precluded,thereby further reducing system costs.

Other variables contributing to system costs, such as thermal managementsystems, represented by a first shaded area 510 in the first column 502and by a second shaded area 512 in the second column 506 may show asmaller relative area in the second column 506 relative to the firstcolumn 502 as a result of additive use to plate uniform layers onto anegative electrode of the IFB. Other elements contributing to overallsystem costs, represented by a third shaded area 514 in the first column502 may represent a larger area, and therefore cost, compared to afourth shaded area 516 in the second column 506.

An example of a method 600 for operating an iron redox flow battery(IFB) system is shown in FIG. 6 . The IFB may include a negativeelectrolyte with a small amount (e.g., millimolar concentration) of anadditive. The additive may be a fatty acid, such as stearic acid, with afunctional end group configured to interact with iron as well as achemically inert carbon tail that does not interact with electroactivespecies in the electrolyte. The IFB may also include a membraneseparator positioned between a negative electrode and a positiveelectrode and configured to moderate exchange of ions between thenegative electrolyte, in contact with the negative electrode, and apositive electrolyte, in contact with the positive electrode. In oneexample, the membrane separator may be an anion exchange membrane (AEM)separator, such as the AEM separator 716 of FIG. 7 , enabling flow ofanions across the AEM.

It will be appreciated that method 600 may be similarly applied to anIFB system using the plating additive and incorporating a cationexchange membrane separator or a microporous substrate in place of theAEM separator. Such alternatives to the AEM separator may enabletransport of chemical species other than anions across the membraneseparator.

At 602, the method includes charging the IFB to store energy via acharging process. In some examples, charging the IFB may be achieved bypulse charging. Application of pulse charging, where a series of voltageor current pulses is applied to the IFB, may result in a reduction of anoverpotential of the negative electrode. The use of pulse charging maymitigate overheating of the battery during charging. The chargingprocess may include applying a current to the IFB to reduce ferrous ironin the negative electrolyte to iron metal and plate the iron metal ontothe negative electrode at 604. Simultaneously, ferrous iron may beoxidized to ferric iron in the positive electrolyte at the positiveelectrode. The additive may interact with the iron so that the ironplates onto the electrode in self-assembled monolayers. The platedmonolayers of iron form an even, uniform coating on the negativeelectrode, allowing for even heat distribution across the negativeelectrode. The plated iron may form a coating greater than 1 cm thick.

Charging the IFB may also include transporting ions, specifically,anions such as chloride, across the AEM separator at 606. The anions mayflow across the AEM separator by an anion exchange mechanism whileexchange of cations across the AEM separator is inhibited. By allowinganions to be exchanged between the positive electrolyte and negativeelectrolyte, a charge balance of the IFB system may be maintained.

In another example where the membrane separator is configured as thecation exchange membrane, cations such as H⁺ may be allowed to flowacross the membrane separator while exchange of anions is inhibited. Inyet another example where the membrane separator is replaced by amicroporous substrate, both cations, such as K⁺, H⁺, and anions, such asCl⁻, FeCl₄ ⁻, are transported. In each of the alternate embodiments ofthe membrane separator, the charge balance of the IFB system may besimilarly maintained.

At 608, the method includes discharging the IFB to provide power to anexternal system. Discharging the IFB may include, at 610, flowing acurrent to the external system from the IFB as iron metal is oxidized toferrous iron at the negative electrode and ferric iron is reduced toferrous iron at the positive electrode. Anions may be transported acrossthe AEM separator at 612 of the method during discharge, providingcharge balance between the positive and negative electrolytes. Byadapting the IFB with the additive in the negative electrolyte and theAEM separator, discharge of the IFB may provide, for example, up to 100hours of energy storage to power the external system.

In this way, a performance of a redox flow battery may be improved whilereducing system costs. The redox flow battery may include an additive inan electrolyte solution in contact with a negative electrode of thebattery. The additive may be a material that interacts withelectroactive cations in the electrolyte and promotes plating of a metalonto the negative electrode in self-assembled monolayers, forming auniform, crack-free coating of metal. The arrangement of the platedmetal in monolayers allows even heating of the negative electrode,simplifying thermal management of the battery, and enables increasedplating thickness without adversely affecting battery performance. Byincreasing the plating thickness, an energy storage capacity of theredox flow battery is enhanced. The technical effect of adapting theredox flow battery with the additive is that a cycling capacity of thebattery is increased while cost per energy unit is decreased.

In one embodiment, a redox flow battery system includes a battery cellwith a positive electrolyte and a negative electrolyte, the positiveelectrolyte in contact with a positive electrode and the negativeelectrolyte in contact with a negative electrode, a plating additiveadded to the negative electrolyte, the plating additive interacting withcations of the negative electrolyte and forming complexes that plateonto the negative electrode in self-assembled monolayers. In a firstexample of the system, during charging of the system, a metal is platedonto the negative electrode and wherein in the presence of the platingadditive, a plating thickness of the metal onto the negative electrodeis greater than when the additive is absent from the redox flow battery.A second example of the system optionally includes the first example,and further includes wherein in the presence of the plating additive, aplating rate of the metal onto the negative electrode is greater thanwhen the additive is absent from the redox flow battery and wherein theplating rate remains greater than when the additive is absent even whena temperature of the redox flow battery system is below a thresholdtemperature. A third example of the system optionally includes one ormore of the first and second examples, and further includes, wherein inthe presence of the plating additive, a uniformity of a coating formedby the plating of the metal onto the negative electrode is increasedrelative to when the additive is absent from the redox flow battery. Afourth example of the system optionally includes one or more of thefirst through third examples, and further includes, wherein in thepresence of the plating additive, a presence of cracks in the coating isreduced relative to when the additive is absent from the redox flowbattery. A fifth example of the system optionally includes one or moreof the first through fourth examples, and further includes, wherein theplating additive has a functional end group at a first end that binds tothe metal and a trailing, chemically inert tail at a second end thatextends away from the metal. A sixth example of the system optionallyincludes one or more of the first through fifth examples, and furtherincludes, wherein the metal, chemically bound by the plating additive,plates onto the negative electrode as a stack of evenly spaced apartmonolayers of the metal that are separated by layers formed of thetrailing, chemically inert tail of the plating additive. A seventhexample of the system optionally includes one or more of the firstthrough sixth examples, and further includes, wherein the platingadditive includes a fatty acid with an electron-rich functional endgroup and a long-chain carbon tail. An eighth example of the systemoptionally includes one or more of the first through seventh examples,and further includes, wherein the plated metal is configured to coat thenegative electrode with a thickness between several mm to over 1 cm.

In another embodiment, a method includes during a charging cycle of theredox flow battery system, plating a metal from an electrolyte solution,the electrolyte solution containing an additive, onto a negativeelectrode to form uniform and crack-free layers of metal, and during adischarging cycle of the redox flow battery system, deplating the metalfrom the negative electrode into the electrolyte solution. In a firstexample of the method, ions are transport across a membrane separatoralong a first direction during the charging cycle of the redox flowbattery system and transporting ions across the membrane along a second,opposite direction during the discharging cycle of the redox flowbattery system and wherein the membrane separator separates a negativeelectrode compartment from a positive electrode compartment of the redoxflow battery system. A second example of the method optionally includesthe first example, and further includes, wherein transporting ionsacross the membrane separator includes transporting anions across ananion exchange membrane separator. A third example of the methodoptionally includes one or more of the first and second examples, andfurther includes, wherein transporting ions across the membraneseparator includes transporting cations across a cation exchangemembrane separator. A fourth example of the method optionally includesone or more of the first through third examples, and further includes,wherein transporting ions across the membrane separator includestransporting ions across a microporous substrate. A fifth example of themethod optionally includes one or more of the first through fourthexamples, and further includes, wherein plating the metal onto thenegative electrode includes forming self-assembled monolayers of themetal onto a surface of the negative electrode. A sixth example of themethod optionally includes one or more of the first through fifthexamples, and further includes, wherein forming self-assembledmonolayers of the metal on the surface of the negative electrodeincludes stacking successive layers of metal atoms along a directionperpendicular to a surface of the negative electrode. A seventh exampleof the method optionally includes one or more of the first through sixthexamples, and further includes, wherein forming self-assembledmonolayers includes forming chemical bonds between the additive and themetal so that the metal is surrounded by chemically inert tails of theadditive.

In yet another embodiment, an iron redox flow battery includes anelectrolyte formed from iron chloride complexes in aqueous solution incontact with a positive electrode and a negative electrode, a chemicalsubstance added to the electrolyte that bonds with iron centers from theiron chloride complexes at a first end of the substance and forms acoating around each of the iron centers and wherein the coating isformed from an inert tail at a second end of the chemical substance. Ina first example of the battery, the inert tail of the chemical substanceis a carbon chain and wherein the coating surrounding each of the ironcenters spaces the iron centers at uniform distances from one another. Asecond example of the battery optionally includes the first example, andfurther includes, wherein the chemical substance is stearic acid.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A redox flow battery system, comprising: an electrolyte solutioncontaining metal cations; and an additive having a functional group at afirst end of the additive and an inert tail at a second end of theadditive, the second end longer than the first end, wherein the additivecauses the metal cations to deposit onto an electrode in monolayersbased on adsorbing of the functional group onto one of the metal cationsand surrounding of the respective metal cation by the inert tail.
 2. Theredox flow battery system of claim 1, wherein the inert tail of theadditive is a hydrocarbon chain that does not interact with the metalcations.
 3. The redox flow battery system of claim 1, wherein themonolayers are formed by self-assembly of a metal of the metal cations,and wherein the monolayers are separated by layers of the additive. 4.The redox flow battery system of claim 3, wherein separation of themonolayers by the layers of the additive reduces a free energy of themetal.
 5. The redox flow battery system of claim 3, wherein theself-assembly of the monolayers of the metal enables formation ofcrack-free layers of the metal on the electrode.
 6. The redox flowbattery system of claim 1, wherein the respective metal cation issurrounded by more than one inert tail of the additive.
 7. The redoxflow battery system of claim 1, wherein the metal cations are ironcations.
 8. The redox flow battery system of claim 1, wherein thefunctional group interacts with the metal cations when the metal cationsare in divalent and trivalent states.
 9. A method for a redox flowbattery system, comprising: responsive to operation of the redox flowbattery system in a charging cycle, plating metal monolayers anelectrode, the metal monolayers including metal cations bound byfunctional groups of additive molecules and with the metal monolayersseparated by layers of the additive molecules; and responsive tooperation of the redox flow battery system in a discharging cycle,deplating the metal monolayers from the electrode.
 10. The method ofclaim 9, wherein the operation in the charging cycle and in thedischarging cycle occurs at ambient temperature.
 11. The method of claim9, wherein the functional groups of the additive molecules arecarboxylates, and wherein the functional groups adsorb onto the metalcations.
 12. The method of claim 9, wherein the additive molecules havelong-chain carbon tails.
 13. The method of claim 12, wherein thelong-chain carbon tails are 0.15 nm to 2.15 nm in length.
 14. The methodof claim 12, wherein a length of the long-chain carbon tails is greaterthan a length of the functional groups.
 15. The method of claim 9,wherein plating the metal monolayers includes stacking the metalmonolayers on the electrode to a thickness of greater than 1 cm.
 16. Amethod for a redox flow battery system, comprising: charging anddischarging the redox flow battery system at ambient temperature usingan electrolyte containing an additive, the additive having a functionalgroup at a first end and an inert tail at a second end, wherein theadditive causes metal cations in the electrolyte to deposit onto anelectrode in monolayers based on adsorption of the functional group ontoone of the metal cations and surrounding of the respective metal cationby the inert tail.
 17. The method of claim 16, wherein the redox flowbattery system is an iron redox flow battery system and the metalcations are iron cations.
 18. The method of claim 16, wherein theelectrode is a negative electrode immersed in the electrolyte, andwherein the electrolyte consists of the metal cations and anions inaddition to the additive.